The present invention relates to wear resistant overlays for use in decorative laminates and floorings and to laminates and flooring prepared therefrom. Decorative laminates have been conventionally made by stacking a plurality of layers of paper impregnated with synthetic thermosetting resins. Normally, the assembly consists of a plurality (for example, three to eight) core sheets made from phenolic resin impregnated Kraft paper, above which lies a decor sheet, usually a print or solid color, impregnated with melamine resin. An overlay sheet is often provided on top of the decor sheet which, in the laminate, is made to be as transparent and as wear-resistant as possible thereby allowing the decor sheet to be seen through the overlay while providing protection for the decor sheet from being damaged by scratches and scuffs. This additional decor sheet, while adding the desired decorative aspects, of course adds to the complexity and cost of the finished laminate.
The overlay sheet, also known as a WROL, can be formed in a multitude of different ways. One conventional manner for forming a suitable WROL comprises forming a sheet of cellulose fibers having a very low basis weight which carries therein and thereon abrasion resistant particles. Conventionally, the cellulose fibers are deposited on the papermachine wire from a first headbox and overcoated with a slurry of mineral particles, such as aluminum oxide, which are deposited from a secondary headbox located relatively closely thereto. The slurry of mineral particles cascades over and through the cellulose fibers thereby causing many of the mineral particles to become embedded in the overlay, as opposed to concentrated near the top of the overlay, where they would be more effective in preventing abrasion to the decor sheet. Additionally, much of the mineral particle slurry does not adhere to the paper and falls to the floor resulting in a waste of materials as well as presenting a cleanliness issue. Addition of the mineral particles in a slurry in this manner is fairly conventional and is generally considered to be a xe2x80x9cwet endxe2x80x9d addition since the grit is applied on the end of the wire prior to the paper being dried.
There have, however, been recent improvements in this method. For example, U.S. Pat. No. 5,820,937 to Crabtree et al., herein incorporated by reference, discloses a similar method except that instead of just dumping the slurry on the paper web using a headbox, the particle slurry is deposited more efficiently using a slot orifice coater such as a curtain coater. But while this method has solved some of the prior art problems (conservation of water, cleanliness issues, waste of materials), some other problems remain unresolved. In particular, while the addition of grit directly to the top of the paper while it is on the wire has been found to improve the abrasion and scuff resistant properties of the end-product laminates, problems have been encountered in attempting to keep the grit particles attached to the top of the paper. Of course, any grit xe2x80x9cfall-offxe2x80x9d that occurs during processing generally results in the decreased abrasion resistance of the overall product. Furthermore, while the addition of a single layer of grit to the WROLs produces improved abrasion resistance over prior art laminates not utilizing grit addition at all, additional abrasion resistance is always desired, especially for laminates to be used in flooring, countertops, etc.
The xe2x80x9cfall-offxe2x80x9d problem discussed herein has been addressed to a certain extent in the past. Specifically, U.S. Pat. No. 4,263,081 to Scher et al. discloses adding microcrystalline cellulose either over the top or underneath the grit to xe2x80x9cbindxe2x80x9d the grit to the paper fibers. The use of microcrystalline cellulose as disclosed in the Scher et al. patent helps to prevent grit migration and fall-off. And while the method described in Scher et al. has proved to be somewhat successful in alleviating some of the above note problems, the use of microcrystalline cellulose by itself has not completely eliminated fall-off problems. Additionally, the extra abrasion resistance provided by the use of microcrystalline cellulose as a binder is relatively negligible.
Accordingly, it would be desirable to have a process for producing wear resistant papers useful for incorporation into decorative laminates that reduce xe2x80x9cfall-offxe2x80x9d problems associated with the addition of grit particles to a negligible level such that decreased abrasion resistance of the finished paper and loss of grit particles are alleviated. Additionally, it would be desirable to have such a process which accomplished this goal while increasing the overall abrasion resistance of the finished papers. Furthermore, it would be desirable to have such a process which produces finished papers relatively cheaply and efficiently and wherein papers produced using such a process would do minimal damage to the process equipment by which such papers were made. Additionally, it would be desirable to have such a process which produces wear resistant papers having the capability of being printed or coated on one side thereof, such capability allowing for the elimination of unnecessary and expensive decor sheets from laminates made incorporating the wear resistant papers. Finally, it would be desirable to produce improved wear resistant papers as described herein that are capable of being directly printed on at least one side thereof, wherein the abrasion resistance of the finished laminate is improved, and wherein the visual properties of the papers, and the finished laminates incorporating the wear resistant papers, such as streaking and clarity are improved or not significantly detrimentally affected.
In accordance with the present invention, an improved wear resistant overlay paper is provided for incorporation into a decorative laminate that may be printed on directly, wherein the overall abrasion resistance of the finished laminate is improved and the visual properties (streaking and clarity) are improved or not significantly detrimentally affected. More specifically, the present invention provides a heretofore unknown wear resistant paper composition comprising multiple paper fiber layers with a mineral particle, or grit, rich interior layer. In one embodiment, the additional fiber layer may be added to the paper by mixing fiber directly with grit slurry and adding the mixture to the base paper layer through a secondary headbox. In a preferred embodiment, the xe2x80x9ctopxe2x80x9d or xe2x80x9cfelt-sidexe2x80x9d fiber layer is a discrete layer that is added over the top of a particle rich interior layer after the grit slurry has been applied to the base paper. In this embodiment, the grit is preferably applied using a curtain or slot orifice coater and the secondary fiber layer is added using a secondary headbox. The addition of the discrete feltside fiber layer provides a smooth surface that may be printed or coated on directly after the forming and drying of the paper and that helps prevent grit migration or xe2x80x9cfall offxe2x80x9d from the wear resistant paper while maintaining or improving the visual properties of the finished laminate incorporating the improved wear resistant paper of the present invention.
As mentioned above, in a preferred embodiment of the present invention, the grit is added to the paper using a curtain or slot orifice coater to improve the retention of the grit on the paper, lower processing costs, and improve the overall quality of the produced wear resistant paper. The addition of grit to wear resistant paper in a manner consistent with a preferred embodiment of the present invention is disclosed in U.S. Pat. No. 5,820,937, the contents of which are herein incorporated by reference.
In yet another preferred embodiment of the present invention, standard aluminum oxide grit is mixed with or replaced by grit, such as aluminum oxide particles, that have been encapsulated in a phenoplast or aminoplast resin. The use of such encapsulated grit provides a finished laminate having the desired abrasion resistant properties while helping to prevent unnecessary wear and tear on papermaking process machinery. The production of suitable encapsulated particles that would be operable for incorporation in the present invention are discussed in U.S. Pat. No. 5,962,134, the contents of which are herein incorporated by reference.
Other objects and advantages will be apparent from the following description, the drawings and the appended claims.